Method and system of transferring a message in a session initiation protocol based communications network

ABSTRACT

Systems and methods of transferring a message in a Session Initiation Protocol (SIP)-based communications network, from a first node to a third node using a second node are provided. In one exemplary embodiment, a method of registering in an Internet Protocol Multimedia Subsystem (IMS) communications network, a User Agent (UA) using a Proxy Call Session Control Function (P-CSCF) entity with a Serving Call Session Control Function (S-CSCF) entity may include determining a first group identifier associated with a plurality of P-CSCF entities. Based on the first group identifier, the method may include selecting a network address corresponding to one of the plurality of P-CSCF entities. Further, the method may include storing subscriber data in a repository associated with the selected P-CSCF entity. Also, the method may include synchronizing, by the selected P-CSCF entity, the subscriber data stored in the repository associated with the selected P-CSCF entity to the remaining P-CSCF entities of the plurality of P-CSCF entities. The UA may be ready to establish and receive SIP sessions through any of the plurality of P CSCF entities.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/497,721, filed Mar. 22, 2012, which is the National Stage ofInternational App. No. PCT/EP2009/063815, filed Oct. 21, 2009, all ofwhich are entitled “METHOD AND SYSTEM OF TRANSFERRING A MESSSAGE IN ASESSION INITIATION PROTOCOL BASED COMMUNICATIONS NETWORK,” and all ofwhich are hereby incorporated by reference as if fully set forth herein.

TECHNICAL FIELD

The invention generally relates to systems and methods of transferring amessage in a Session Initiation Protocol (SIP)-based communicationsnetwork and, in particular, to an Internet Protocol (IP) MultimediaSubsystem (IMS) network.

BACKGROUND

Voice Over Internet Protocol (VOIP) networks, notably Internet Protocol(IP) Multimedia Subsystem (IMS) networks as defined by the 3^(rd)Generation Partnership Project (3GPP), are designed around a set offunctional entities, within this document also referred to as nodes,each performing particular tasks.

Most of the nodes in a VOIP network, such as an IMS network, aredeployed in redundant or geographically distributed method, as is commonin telecommunication networks. Reasons include: load sharing (e.g. dueto the number of subscribers in the network or due to the call density),geographic distribution (e.g. because of the size of the geographicalarea to cover) and reliability (e.g. switching over to another node inthe case of temporary node outage/link failure or in the case of nodemaintenance). E.g. when a Serving Call Session Control Function (S-CSCF)entity is temporarily not operational, due to failure or due to plannedmaintenance, subscribers may be registered in another S-CSCF which willtake over the tasks of the inoperational S-CSCF. Likewise, when a P-CSCFis temporarily not operational, the subscriber would have to registerwith another P-CSCF.

In one known embodiment, the Home Subscriber Server HSS accepts arequest for subscription data from one S-CSCF, whilst the HSS has markedthe subscriber as currently being registered with another S-CSCF.

In another known embodiment, frequent re-registrations are applied.Re-registrations are needed to keep a binding between a public useridentity and a contact address (or simply ‘binding’) in S-CSCF alive.When a Session Initiation Protocol User Agent (SIP-UA) performsre-registration, the registration-related data in the IMS network is(re-)synchronized at that moment. If due to network problems thesubscriber's registration had to be moved to another S-CSCF, thenresulting from the re-registration, the SIP-UA, P-CSCF, S-CSCF and HSSare again synchronized. By configuring the S-CSCF to instruct a SIP-UAto apply very frequent re-registrations, the time that a subscriberwould be non-contactable due to node outage is minimized, at the expenseof increased network signalling, which, in commercial IMS deployments,amounts to a large percentage of the total SIP signalling.

When a P-CSCF becomes inoperational, (regular) re-registration by aSIP-UA would not resolve the problem. The SIP-UA will attempt toregister with the same P-CSCF as ‘discovered’ prior to initialregistration.

SUMMARY

The inventor realised that the known VOIP networks, in particular theIMS networks, may be improved. More in general, it is an object of theinvention to provide an improved method of transmitting a message in aSIP based network.

According to the invention is provided a method of transferring amessage in a Session Initiation Protocol (SIP) based communicationsnetwork, from a first node to a third node via a second node, comprisingthe steps of storing, in a repository associated with the first node, agroup identifier associated with a plurality of second nodes, deriving,by the first node, on the basis of the group identifier a networkaddress corresponding to one second node of the plurality of secondnodes, and transmitting the message to the third node via the secondnode derived by the first node.

Thus, the first node is not obliged to send the message to the thirdnode via one predetermined second node, but the first node may derive,e.g. select, one of a plurality of second nodes.

The present invention may result in improved network robustness.Specifically, it may remove the necessity for a SIP subscriber to remainregistered with a particular Proxy Call Session Control Function(P-CSCF) entity as second node and/or with a particular Serving CallSession Control Function (S-CSCF) entity as second node. Node failure ina P-CSCF entity or S-CSCF entity will be handled by another P-CSCFentity or S-CSCF entity taking over responsibility for new SIP sessions,for new SIP transactions or for SIP response messages. Shared user datawithin a group of P-CSCF entities and within a group of S-CSCF entitiesmay facilitate above-referred taking over.

Thereto, it is possible that the method comprises synchronising amongthe plurality of second nodes, data relating to the first and/or thirdnode, e.g. data relating to a relationship between the first and thesecond node and/or between the third and the second node, such as userdata. Thus, each second node of the plurality of second nodes possessesup-to-date data while forwarding the message received from the firstnode towards the third node. When a second node commences the processingof a message, all second nodes within the group may have up-to-datedata. When the processing of the message results in change of data, thenthe data could be synchronised with, e.g. transmitted to, the othersecond nodes prior to forwarding the message.

It is possible that the step of deriving comprises performing a DomainName Server (DNS) query on the basis of the group identifier. The DNSmay for example return a list of Internet Protocol (IP) addressescorresponding to the second nodes of the plurality of second nodes. Thefirst node may then select one of the IP addresses.

It is possible that the message contains the group identifier, and themethod further comprising the steps of deriving, by the third node, onthe basis of the group identifier a network address corresponding to onesecond node of the plurality of second nodes, and transmitting, inresponse to receiving the message, a response from the third node to thefirst node via a second node selected by the third node. The first nodeor the second node may enter the group identifier in the message, e.g.in a header of the message. Thus, the group identifier may bedistributed from the first node or second node to the third node. Itwill be appreciated that the response may be transmitted via a differentsecond node than the message.

It is possible that the plurality of second nodes is a plurality ofuser-to-network proxy entities. The plurality of second nodes may be aplurality of Proxy Call Session Control Function (P-CSCF) entities. Thegroup identifier may then be stored in a repository associated with auser equipment, for instance a Session Initiation Protocol User Agent(SIP-UA). The SIP-UA may then transmit the message to e.g. a ServingCall Session Control Function (S-CSCF) entity via one of the pluralityof P-CSCF entities. It will be appreciated that herein the userequipment is also referred to as a node.

It is possible that the plurality of second nodes is a plurality ofregistrars or service nodes. The plurality of second nodes may be aplurality of S-CSCF entities. The group identifier may then e.g. bestored in a repository associated with a user-to-network proxy, such asa P-CSCF entity or in a repository associated with the SIP-UA). TheP-CSCF entity may then transmit the message to a recipient via one ofthe plurality of S-CSCF entities.

The invention also relates to a method of performing a SessionInitiation Protocol (SIP) session in an Internet Protocol MultimediaSubsystem (IMS) communications network, between a User Agent (UA) and anode, via a selected Proxy Call Session Control Function (P-CSCF) entityand a selected Serving Call Session Control Function (S-CSCF) entity,comprising the steps of storing, in a repository associated with theUser agent (UA), a first group identifier associated with a plurality ofProxy Call Session Control Function (P-CSCF) entities, and selecting onthe basis of the first group identifier a network address correspondingto one P-CSCF entity of the plurality of P-CSCF entities, and/or storingin a repository associated with the User agent (UA) or the selectedP-CSCF entity, a second group identifier associated with a plurality ofServing Call Session Control Function (S-CSCF) entities, and selectingon the basis of the second group identifier a network addresscorresponding to one S-CSCF entity of the plurality of S-CSCF entities,and transmitting a message to the node via the selected P-CSCF entityand the selected S-CSCF entity. Thus, the message may be transmittedfrom the User Agent to the node via any one of the P-CSCF entities ofthe plurality of P-CSCF entities, and/or via any one of the S-CSCFentities of the plurality of S-CSCF entities.

It is possible that the selected P-CSCF entity of the plurality ofP-CSCF entities is used throughout the SIP session. It is also possiblethat the selected S-CSCF entity of the plurality of S-CSCF entities isused throughout the SIP session. It is also possible that a P-CSCFentity is selected per transaction within the SIP session. It is alsopossible that a S-CSCF entity is selected per transaction within the SIPsession. It is also possible that a P-CSCF entity is selected permessage within a single transaction within the SIP session. It is alsopossible that a S-CSCF entity is selected per message within a singletransaction within the SIP session.

It is possible that the selected P-CSCF entity of the plurality ofP-CSCF entities synchronises subscriber data stored in a repositoryassociated with that selected P-CSCF entity to the remaining P-CSCFentities of the plurality of P-CSCF entities, and/or the selected S-CSCFentity of the plurality of S-CSCF entities synchronises subscriber datastored in a repository associated with that selected S-CSCF entity tothe remaining S-CSCF entities of the plurality of S-CSCF entities.

It will be appreciated that if the selected P-CSCF entity of theplurality of P-CSCF entities is selected for a shorter duration, e.g.per transaction, the frequency of synchronisation may increase, but thesensitivity to node failure decreases.

It is possible that the message comprises the first group identifierand/or the second group identifier, for instance in a Via header and/ora Record-route header.

In an embodiment, a response to the message, within the context of asingle transaction, is transmitted from the node to the User Agent, viaa selected Proxy Call Session Control Function (P-CSCF) entity and aselected Serving Call Session Control Function (S-CSCF) entity, whereinthe method further comprises selecting, e.g. by the node, on the basisof the first group identifier a network address corresponding to oneP-CSCF entity of the plurality of P-CSCF entities, and/or selecting,e.g. by the node or by the P-CSCF entity, on the basis of the secondgroup identifier a network address corresponding to one S-CSCF entity ofthe plurality of S-CSCF entities. Hence, the response may also betransmitted via any one of the P-CSCF entities of the plurality ofP-CSCF entities, and/or via any one of the S-CSCF entities of theplurality of S-CSCF entities.

The invention also relates to a SIP based communications network,comprising a first node, a second node and a third node, wherein thefirst node has a repository associated therewith, the repositorycomprising a group identifier associated with a plurality of secondnodes, wherein the first node is arranged for deriving, on the basis ofthe group identifier, a network address corresponding to one second nodeof the plurality of second nodes, and wherein the network is arrangedfor transmitting a message to the third node via the selected secondnode.

In the event that the SIP based communications network is an InternetProtocol Multimedia Subsystem (IMS) communications network, the firstnode may for instance be a User Agent. Then the third node may be arecipient node. Also, the plurality of second nodes may be a pluralityof P-CSCF entities or a plurality of S-CSCF entities. It is alsopossible that the plurality of second nodes comprises two or more groupsof second nodes, such as a plurality of P-CSCF entities and a pluralityof S-CSCF entities, wherein the message may be transmitted from thefirst node to a second node of one group, then to a second node ofanother group, and then to the third node.

The invention also relates to an Internet Protocol Multimedia Subsystem(IMS) communications network comprising a User Agent (UA), a selectedProxy Call Session Control Function (P-CSCF) entity, a selected ServingCall Session Control Function (S-CSCF) entity and a recipient node,wherein the User Agent has a repository associated therewith, storing afirst group identifier associated with a plurality of Proxy Call SessionControl Function (P-CSCF) entities, and/or a second group identifierassociated with a plurality of Serving Call Session Control Function(S-CSCF) entities, wherein the User Agent is arranged for selecting onthe basis of the first group identifier a network address associatedwith one P-CSCF entity of the plurality of P-CSCF entities, and/orwherein the User Agent or the selected P-CSCF entity is arranged forselecting on the basis of the second group identifier a network addressassociated with one S-CSCF entity of the plurality of S-CSCF entities,and wherein the network is arranged for transmitting a message to therecipient node via the selected P-CSCF entity and the selected S-CSCFentity.

Optionally, the plurality of P-CSCF entities is arranged forsynchronising subscriber data among the plurality of P-CSCF entities. Itis also possible that the plurality of S-CSCF entities is arranged forsynchronising subscriber data among the plurality of S-CSCF entities.

The invention also relates to a network node for transmitting a messageto a recipient node via an intermediate node in a Session InitiationProtocol, SIP, based communications network, wherein the node has arepository associated therewith arranged for comprising a groupidentifier associated with a plurality of intermediate nodes, whereinthe node is arranged for deriving, e.g. selecting, on the basis of thegroup identifier, a network address corresponding to one intermediatenode of the plurality of intermediate nodes; and wherein the node isarranged for transmitting a message to the recipient node via theintermediate node derived by the node. Such network node may forinstance be a User Agent, e.g. a user equipment, or a P-CSCF entity.

The invention also relates to a network node for receiving a messagefrom a transmitting node and forwarding the message to a recipient nodein a Session Initiation Protocol, SIP, based communications network,wherein the node has a repository associated therewith arranged forcomprising a group identifier associated with a plurality of such nodes,wherein the node is arranged for synchronising subscriber data among theplurality of intermediate nodes. Such node may for instance be a P-CSCFentity or a S-CSCF entity.

The invention also relates to a Session Initiation Protocol message inan Internet Protocol Multimedia Subsystem network, comprising a groupidentifier associated with a plurality of Proxy Call Session ControlFunction entities, for instance in a Via header and/or a Record-routeheader, and/or a group identifier associated with a plurality of ServingCall Session Control Function entities, for instance in a Via headerand/or a Record-route header.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further elucidated by means of non-limitingexamples referring to the drawing, in which

FIG. 1 shows an example of a schematic representation of a networkaccording to the invention;

FIG. 2 shows an example of a schematic representation of logicalrelationships between nodes in an IMS network according to theinvention;

FIG. 3 shows an example of subscriber registration according to theinvention;

FIG. 4 shows an example of Session establishment by a User Agentaccording to the invention;

FIG. 5 shows an example of Session establishment towards a User Agentaccording to the invention; and

FIG. 6 shows an example of call establishment by a User Agent withtransaction based synchronisation according to the invention.

DETAILED DESCRIPTION

Voice Over Internet Protocol (VOIP) networks, notably Internet Protocol(IP) Multimedia Subsystem (IMS) networks as defined by the 3^(rd)Generation Partnership Project (3GPP), are designed around a set offunctional entities, also referred to as nodes herein, each performingparticular tasks. These nodes can be used in both prior art networks andin a network according to the invention.

Below is given a (non-exhaustive) list of typical nodes in a Voice OverIP network. Also is described their usual use in prior art networks.

-   -   User-to-network proxy entity; the user-to-network proxy entity        is functionally connected between a user equipment and the IMS        network; it resides at the edge of the IMS network. Between the        user-to-network proxy entity and the user equipment, resides the        Access Network. Some of the tasks of the user-to-network proxy        entity include: ensuring integrity of signalling messages        arriving from the user equipment or being sent to the user        equipment, asserting data received from the user equipment,        applying signalling encryption and signalling compression (e.g.        for wireless connections) for data connection with the user        equipment.    -   Registrar; when a VOIP subscriber registers as user in the VOIP        network, she deposits her public user identity and her contact        address, typically an Internet Protocol (IP) address (optionally        with a user part), in the registrar. This enables calls being        established towards that VOIP user, i.e. towards that public        user identity, to be offered to this user's user equipment. The        registrar resides in the IP network (sub-network) of the VOIP        operator. The combination of public user identity and contact        address is known as a binding. The registration procedure may        include authentication.    -   Inbound proxy; when a call is established towards a VOIP        subscriber, the call will enter the VOIP network via the inbound        proxy. The inbound proxy queries a Location server to get an        indication of the registrar that holds the binding of this        subscriber. The inbound proxy is also used to obtain the address        of the registrar that shall be used for subscriber registration.    -   Location server; the location server keeps track of the        registrar in which a subscriber is registered. The selection of        registrar is done by the VOIP network, not by the user.    -   Network-to-network proxy entity; when a VOIP call is established        that breaks out to or breaks in from another network, then the        signalling traverses a network-to-network proxy entity. If the        other network uses another signalling protocol, e.g. ISDN User        Part (ISUP) for the Integrated Services Digital Network (ISDN),        then the network-to-network proxy has to perform signalling        conversion. For breaking out from VOIP network to another        VOIP/non-VOIP network, a ‘gateway selector’ may be used; this        gateway selector selects the Network-to-network proxy entity or        gateway to be used for the breakout.

The above-described nodes for the VOIP network form part of the Controlplane, which handles signalling for subscriber registration, theestablishment of calls and other tasks. Besides control plane nodes, theVOIP network contains nodes that form part of the User plane. The Userplane carries the media for a call, e.g. Pulse Code Modulation (PCM(ITU-T G.711)) encoded voice.

A signalling protocol commonly used in contemporary VOIP networks is theSession Initiation Protocol (SIP). SIP is defined by the Internetengineering task force (IETF; www.ietf.org) and is published in RFC 3261(RFC=Request for comments; is a naming convention for IETF standards).Many extensions and enhancements to SIP are introduced, each describedin a separate IETF RFC. For carrying voice and other real-time mediasuch as video over the user plane, the Real-time Transport Protocol(RTP) may be used, which is published in RFC 3550. Besides SIP and RTP,Diameter protocol is used between designated nodes in the VOIP network,for various processes, such as registration and terminating callestablishment. Diameter base protocol is published in RFC 3588.

The IP Multimedia Subsystem (IMS) has adopted various protocols fromIETF, such as SIP, RTP and Diameter. The architecture of an IMS networkincludes nodes that resemble the nodes that were described for the VOIPnetwork, such as (list is not exhaustive):

SIP user agent (SIP-UA). The SIP-UA is the user equipment, such as amobile communications device or a wireline communications device.

Proxy Call Session Control Function (P-CSCF) entity. The P-CSCF entityserves as user-to-network proxy entity. A subscriber is assigned to aP-CSCF during IMS registration. Hereto, the subscriber (the userequipment) ‘discovers’ a P-CSCF entity during registration. Variousmethods are defined for the subscriber to discover his P-CSCF. Theselection of P-CSCF may e.g. be done by Dynamic Host ConfigurationProtocol (DHCP) associated with the Access Network or may be setmanually in the user equipment, e.g. by the user. Once a subscriber isregistered with a particular user equipment in the IMS network, all callrelated signalling between this subscriber user equipment and networkhas to traverse that particular proxy entity, i.e. the P-CSCF.

Serving Call Session Control Function (S-CSCF) entity. The S-CSCF entityperforms the role of registrar. The subscriber places his contactaddress, e.g. IP address, in a repository associated with the S-CSCFentity, enabling the S-CSCF entity to forward an incoming call to thatsubscriber. The S-CSCF entity also acts as Service Node. When a call isestablished from or to an IMS subscriber, the call signalling traversesthe S-CSCF entity. The S-CSCF entity may then apply services for thiscall from/to the subscriber. These services may be performed in theS-CSCF entity or in an external node, such as a SIP Application Server(SIP-AS). As the subscriber will be registered with a particular S-CSCFentity, all call related signalling to/from the end-user has to behandled by this particular S-CSCF entity.

Interrogating Call Session Control Function (I-CSCF) entity. The I-CSCFentity is the inbound proxy for the IMS network. When a call enters theIMS network, the I-CSCF entity queries a Home Subscriber Server (HSS) toobtain the address of the S-CSCF entity where the subscriber isregistered, i.e. which holds the subscribers binding (contact address).In addition, when a subscriber registers in the IMS network, the I-CSCFentity assists in sending the registration message to an appropriateS-CSCF entity (if not already registered) or to the currently usedS-CSCF entity (if already registered). I-CSCF does not hold subscriberdata.

Home Subscriber Server (HSS). The HSS holds subscriber records,containing IMS subscription data. When a subscriber registers in the IMSnetwork, the HSS is updated as to which S-CSCF entity is now holding thesubscriber's binding. The HSS will, in turn, provide IMS subscriptiondata to the S-CSCF entity. Some of this subscription data may beforwarded from the S-CSCF entity to the P-CSCF entity. And some of thatdata may be forwarded further from the P-CSCF entity to the userequipment.

Interconnect Border Control Function (IBCF) entity. The IBCF entityinterconnects two IMS networks. In that role, it may perform tasks suchas protocol adaptation (e.g. IPv4-IPv6), service restriction, resourceusage restriction (bandwidth control), topology hiding (shielding IPaddresses and host names) and interconnect/transit accounting. The IBCFentity does not hold subscriber data.

Media Gateway Control Function (MGCF) entity. The MGCF entity performssignalling connection between the IMS network and Circuit switched (CS)networks, such as ISDN or GSM. The MGCF entity does not hold subscriberdata.

Most of the nodes in a VOIP network, such as an IMS network, aredeployed in redundant or geographically distributed method, as is commonin telecommunication networks. Reasons include: load sharing (e.g. dueto the number of subscribers in the network or due to the call density),geographic distribution (e.g. because of the size of the geographicalarea to cover) and reliability (e.g. switching over to another node inthe case of temporary node outage/link failure or e.g. in the case ofnode maintenance).

As described above, a subscriber will be registered in the prior art IMSnetwork through a particular P-CSCF entity and will be registered in aparticular S-CSCF entity. Hence, in order to guarantee correct handlingof services in the network, it is required that nodes in the IMS networkremain synchronized. One node needs to know which other node is holdingsubscriber data related to the same subscriber. One can visualize thisby depicting registration relationships in the prior art IMS network.These relationships are static, not related to communication activity atany moment. The following registration relationships may be defined:

(1) between SIP-UA and P-CSCF entity:

-   -   The SIP-UA stores the network address of the P-CSCF entity        (resulting from P-CSCF discovery procedure), and possibly        security association related data and/or signalling compression        related data;    -   The P-CSCF entity stores the public identity of the SIP-UA (as        received from the SIP-UA), and possibly security association        related data and/or signalling compression related data.

(2) between SIP-UA and S-CSCF entity:

-   -   The SIP-UA stores the S-CSCF network address (also referred to        as ‘Service-route’), as received from the S-CSCF entity in a SIP        200 Ok response to a SIP Register request;    -   The S-CSCF entity stores the contact address of the SIP-UA, as        well as terminal capabilities, if applicable, both as received        from the SIP-UA in the SIP Register request.

(3) between SIP-UA and HSS:

-   -   The SIP-UA stores subscription data of the subscriber, received        from the HSS, via the S-CSCF entity and the P-CSCF entity, such        as a list of Public user identities that may be used by this        SIP-UA for establishing and receiving calls. The HSS does not        store data received from the SIP-UA, so that the relation        between the SIP-UA and HSS is unidirectional.

(4) between P-CSCF and S-CSCF:

-   -   The P-CSCF entity stores the S-CSCF entity network address        (‘Service-route’), as received from the S-CSCF entity;    -   The S-CSCF entity stores the P-CSCF entity network address (also        referred to as ‘Path’), as received from the P-CSCF entity.

(5) between P-CSCF and HSS:

-   -   The P-CSCF entity stores subscription data of the subscriber        received from the HSS, via the S-CSCF entity, such as charging        parameters or the aforementioned list of Public user identities.        The HSS does not store data received from the P-CSCF entity, so        that the relation between the P-CSCF entity and the HSS is        unidirectional.

(6) between S-CSCF and HSS:

-   -   The S-CSCF entity stores subscriber subscription data, received        from the HSS. The S-CSCF entity does not store the HSS address.    -   The HSS stores the network address of the S-CSCF entity.

Next, an example of a network according to the invention will bedescribed. It will be appreciated that the network according to theinvention in certain aspects relies on the above described relationshipsbetween nodes in the prior art network.

FIG. 1 shows an example of a network 1 according to the invention. Inthis example, the network comprises a Session Initiation Protocol UserAgent (SIP-UA) 2, a plurality of Proxy Call Session Control Function(P-CSCF) entities 4.i (i=1, 2, 3, . . . ), an Interrogating Call SessionControl Function (I-CSCF) entity 6 and a plurality of Serving CallSession Control Function (S-CSCF) entities 8.j (j=1, 2, 3, . . . ). Inthis example, the network also comprises a Home Subscriber Server (HSS)10.

In this example, the SIP-UA 2 comprises retrieving means 18, arepository 20, input/output means 22, control means 24 and selectionmeans 26. In this example, the P-CSCF entities 4.i each compriseinput/output means 28, control means 30, distribution means 31,retrieving means 33, and optionally a repository 32. In this example,the I-CSCF entity 6 comprises input/output means 34, control means 36,retrieving means 38, optional selection means 40 and a repository 42. Inthis example, the S-CSCF entities 8,j each comprise input/output means44, control means 46, retrieving means 48, selection means 50, arepository 52 and distribution means 54.

In this example, for registration the SIP-UA 2 does not discover asingle P-CSCF entity network address, but instead discovers a P-CSCFgroup identifier. The P-CSCF group identifier is associated with theplurality of P-CSCF entities 4.i. The P-CSCF group identifier may forinstance be a domain name.

The actual registration process of the IMS subscriber (SIP-UA) is donethrough a selected one P-CSCF entity of the plurality of P-CSCF entities4.i. In this example, the SIP-UA contacts a Domain Name Server (DNS) 16for retrieving a network address corresponding to the selected P-CSCFentity. Thereto, the SIP-UA may use the retrieving means 18. Here DNSname resolving of the P-CSCF group identifier by the SIP-UA yields thenetwork address, e.g. IP address, of one particular P-CSCF through whichthe registration shall be done. This particular P-CSCF is used for theregistration process. It will be appreciated that the SIP-UA now hasderived the network address of one P-CSCF entity of the plurality ofP-CSCF entities on the basis of the P-CSCF group identifier. It will beappreciated that DNS may return multiple network addresses, e.g.multiple IP addresses, related to multiple P-CSCF entities of theplurality of P-CSCF entities 4.i. The SIP-UA 2 may in such case selectone network address for the registration process using the selectionmeans 26. It will be appreciated that the SIP-UA now has selected thenetwork address of one P-CSCF entity of the plurality of P-CSCF entitieson the basis of the P-CSCF group identifier.

Regular IP address usage may be applied here, entailing that when thesending of the SIP registration message to one IP address fails (becausethe P-CSCF related to that IP address is not operational at thatmoment), the SIP-UA may attempt to send the SIP registration message toanother IP address. Such method to try a second IP address in the casethat the first IP address fails, is standard IP methodology, known inthe art.

It will be appreciated that the DNS 16 may apply a rule when returningmultiple network addresses as a response to a P-CSCF group identifier.The DNS may for instance return a list of network addresses, the orderof which is adjusted for present load of the P-CSCF entities associatedwith the network addresses in the list. Alternatively, the DNS 16 mayvary, e.g. cycle, the order of the addresses in the list. This may beuseful in that nodes often simply use the first address of the list ofaddresses returned. Alternatively, or additionally, the DNS may append aweight factor to each address returned, for e.g. reflecting the currentload for these nodes.

The Path header that the P-CSCF entity includes in the SIP Register maybe the P-CSCF group identifier. The selected P-CSCF entity 4.i forwardsthe Registration message to an I-CSCF entity 6 which selects a S-CSCFentity 8.j, as in prior art networks. In this example, the I-CSCF entityselects a S-CSCF group identifier for this subscriber, rather than theaddress (host name) of a single S-CSCF entity. The S-CSCF groupidentifier is associated with the plurality of S-CSCF entities 8.j.

The actual registration process is done through a selected S-CSCF entityof the plurality of S-CSCF entities 8.j. In this example, DNS nameresolving by the selected I-CSCF entity, yields the network address ofthe selected S-CSCF entity of the plurality of S-CSCF entities 8.jthrough which the registration shall be done. It will be appreciatedthat the DNS 16′ queried by the I-CSCF entity may be the same DNS asqueried by the SIP-UA, or another DNS. It will be appreciated that DNSmay return multiple network addresses, e.g. multiple IP addresses,related to multiple S-CSCF entities of the plurality of S-CSCF entities.The I-CSCF entity may select one IP address using the selection means40. If the selected S-CSCF entity is not operational, the I-CSCF entitymay try to send the SIP Registration message to another S-CSCF entity ofthe plurality of S-CSCF entities, of which it may also have received theIP address from DNS. It will further be appreciated that the I-CSCFentity may have cached, in the repository 42, the IP addresses of theS-CSCF entities within the plurality of S-CSCF entities. So long as thevalidity period of these addresses has not expired, the I-CSCF entitywould not have to perform a DNS query.

The selected P-CSCF entity receives a Service-route from the selectedS-CSCF entity, in the 200 Ok message. This Service-route may be a groupService-route, rather than a Service-route related to a single S-CSCFentity. The Service-route may comprise the S-CSCF group identifier. TheP-CSCF entity may store this group Service-route.

In this example, the SIP-UA 2 stores the (discovered) P-CSCF groupidentifier in the repository 20 and uses that every time it initiates aSIP session towards the network or initiates a stand-alone transaction.Similarly, the SIP-UA stores the S-CSCF group identifier that itreceives in the 200 Ok for the registration in the repository 20. EachSIP session establishment or stand-alone transaction may be done througha different P-CSCF entity within the plurality of P-CSCF entities 4.iand through a different S-CSCF entity within the plurality of S-CSCFentities 8.j.

In a preferred embodiment, the data related to (a) the relation betweenthe selected P-CSCF entity and the SIP-UA, (b) the relation between theselected P-CSCF entity and the selected S-CSCF entity and/or (c) therelation between the selected P-CSCF entity and the HSS is not keptsolely within the selected P-CSCF entity through which the registrationis done, but is distributed within the plurality of P-CSCF entities.Thereto, the P-CSCF entities 4.i may use the distribution means 31.Resulting from this data distribution, SIP session establishment andstand-alone SIP transactions may be done through any of the P-CSCFentities of the plurality of P-CSCF entities.

Data synchronisation between the P-CSCF entities is schematicallydepicted in FIG. 1 by arrow 12.

In a preferred embodiment, the data related to (a) the relation betweenthe selected S-CSCF entity and the SIP-UA, (b) the relation between theselected S-CSCF entity and the selected P-CSCF entity and/or (c) therelation between the selected S-CSCF entity and the HSS is not keptsolely within the selected S-CSCF through which the registration isdone, but is distributed within the plurality of S-CSCF entities.Thereto, the S-CSCF entities 8.j may use the distribution means 54.Resulting from this data distribution, SIP session establishment andstand-alone SIP transactions may be done through any of the S-CSCFentities within the plurality of S-CSCF entities.

Data synchronization between the S-CSCF entities is schematicallydepicted in FIG. 1 by arrow 14.

In this example, the HSS 10 stores the S-CSCF group identifier, which itreceives from the selected S-CSCF entity through which the subscriberhas done registration. When the HSS has to steer a SIP Invite requestmessage or a SIP Register request message to a S-CSCF entity, the SIPInvite or SIP Register will be sent to any of the S-CSCF entities in theplurality of S-CSCF entities.

When the HSS has to update the subscriber data stored in the S-CSCFentity repository 52, it may perform the updating towards any of theS-CSCF entities within the group of S-CSCF entities allocated to thissubscriber. The S-CSCF entity receiving the updated data takes care thatthe updated data is distributed within the plurality of S-CSCF entitiesusing the distribution means 54.

Resulting from the registration method as described above, therelationships in the IMS network for a particular subscriber will be asfollows.

The SIP-UA has a relation with a plurality of P-CSCF entities (by virtueof storing the P-CSCF group identifier in the repository 20) as well asa relation with a plurality of S-CSCF entities (by virtue of storing theS-CSCF group identifier in the repository 20). A SIP session orstand-alone transaction initiated by the SIP-UA may be establishedthrough any of the P-CSCF entities and any one of the S-CSCF entitieswithin their respective plurality. It is noted that security andcompression related data stored by the SIP-UA, needed for the dataconnection with a P-CSCF over (public) internet shall be such that theSIP-UA can communicate (exchange SIP messages) with any one of theP-CSCF entities within the plurality of P-CSCF entities.

Each P-CSCF entity within the plurality of P-CSCF entities has, for thissubscriber, a relation with this subscriber's plurality of S-CSCFentities (e.g. by virtue of storing the S-CSCF group identifier, in therepository 32). It is noted that security and compression related datastored by each P-CSCF entity, needed for the data connection with theSIP-UA over (public) internet shall be such that each P-CSCF entity ofthe plurality of P-CSCF entities can communicate (exchange SIP messages)with the SIP-UA.

Each S-CSCF within the plurality of S-CSCF entities has, for thissubscriber, a relation with this subscriber's plurality of P-CSCFentities (e.g. by virtue of storing the P-CSCF group identifier in therepository 52). A SIP session or stand-alone transaction destined forthe SIP-UA may be established through any of the S-CSCF entities and anyof the P-CSCF entities within their respective plurality.

The HSS maintains, for this subscriber, a relation with thissubscriber's plurality of S-CSCF entities (e.g. by virtue of storing theS-CSCF group identifier).

An incoming SIP session or stand-alone transaction as well as subscriberdata update may be directed towards any one of the S-CSCF entities inthe plurality of S-CSCF entities.

FIG. 2 depicts the relationships that may exist after subscriberregistration (registration relationships), in accordance with the aboveregistration.

FIG. 3 shows an example of registration by the SIP-UA, in accordancewith the invention. In step (1), the SIP-UA 2 discovers the P-CSCFaddress being the P-CSCF group identifier(sip:p-cscf-cluster1.provider.com) identifying a group, here identifiedas cluster 1, of P-CSCF entities and uses it as Route header in SIPRegister. The SIP-UA applies DNS resolving (not depicted) and obtains anIP address associated with the P-CSCF group identifier. In this example,the IP address is the IP address of P-CSCF1. The SIP-UA sends a SIPRegister towards this IP address.

In step (2), P-CSCF1 recognizes the address in the Route header(sip:p-cscf-cluster1.provider.com) as its own group identifier, soremoves this Route header. P-CSCF1 adds the group proxy address as Pathheader (Path:<sip:term@p-cscf-cluster1.provider.com;Ir>).

In step (3), the I-CSCF entity contacts the HSS and receives S-CSCFcapabilities. Based on these capabilities, the I-CSCF entity selects thegroup of S-CSCF entities identified as cluster 1(sip:s-cscf-cluster1.provider.com) and uses that address as Routeheader. The I-CSCF entity applies DNS resolving and, in this exampleretrieves/selects the IP address of S-CSCF2.

In step (4), S-CSCF2 contacts the HSS and receives authorization data.The HSS stores the address, i.e. the group identifiers-cscf-cluster1.provider.com, of the S-CSCF cluster that S-CSCF2 belongsto.

In step (5), S-CSCF2 stores the data received from the HSS. S-CSCF2 thensynchronises subscriber data with the other S-CSCF entities within theplurality of S-CSCF entities, in this case with S-CSCF1.

All S-CSCF entities in the plurality of S-CSCF entities are now preparedto receive the second registration message.

In step (6), when P-CSCF1 receives the 401 Unauthorized, it stores datacontained in this response and then synchronises subscriber data withthe other P-CSCF entities within the plurality of P-CSCF entities, inthis case with P-CSCF2.

All P-CSCF entities in the plurality of P-CSCF entities are now preparedto receive the second registration message.

In step (7), the SIP-UA sends the second Register message and uses againsip:p-cscf-cluster1.provider.com as the Route header. This time, DNSresolving leads to sending the SIP Register message to P-CSCF2 (but itcould also have been P-CSCF1).

In step (8), P-CSCF2 recognizes the address in the Route header(sip:p-cscf-cluster1.provider.com) as its own group address, so removesthis Route header. P-CSCF2 adds the group proxy address as Path header.P-CSCF2 has, due to data synchronization initiated by P-CSCF1, dataavailable for the security relation with the SIP-UA.

In step (9), the I-CSCF entity (which could be another I-CSCF entitythan used in the first registration request) contacts the HSS. The HSSinforms the I-CSCF entity that the subscriber shall be registered withs-cscf-cluster1.provider.com. DNS resolving by the I-CSCF entity leadsto the SIP Register being sent to S-CSCF1 (but it could also have beenS-CSCF2). S-CSCF1 has, due to data synchronization initiated by S-CSCF2,data available for authenticating the subscriber. S-CSCF1 informs theHSS about the subscriber assignment.

In step (10), the HSS stores the S-CSCF entity address in the form ofthe S-CSCF group identifier (s-cscf-cluster1.provider.com) and providesS-CSCF1 with subscription data. The HSS is prepared to direct SIPsession establishment requests to any of the S-CSCF entities in theplurality of S-CSCF entities.

In step (11), S-CSCF1 stores the data, as normal, and synchronisessubscriber data with the other S-CSCF entities in the plurality ofS-CSCF entities, in this case with S-CSCF2. The service-route providedtowards the calling party identifies the plurality of S-CSCF entities.All S-CSCF entities in the plurality of S-CSCF entities are now preparedto establish SIP sessions for the subscriber.

In step (12), P-CSCF2 stores the service-route and optionally othersubscriber data (e.g. P-associated-URI) and then synchronises subscriberdata with the other P-CSCF entities in the plurality of P-CSCF entities,in this case with P-CSCF1. All P-CSCF entities in the plurality ofP-CSCF entities are now prepared to establish SIP sessions for thisSIP-UA of this subscriber.

In step (13), the SIP-UA receives the 200 Ok and stores theService-route and other subscriber data. The SIP-UA is now ready toestablish and receive SIP sessions through any of the P-CSCF entities inthe plurality of P-CSCF entities and through any of the S-CSCF entitiesin the plurality of S-CSCF entities.

In this example, the distribution of data between logical entities inthe IMS network is done through existing SIP signalling methodology. Forexample, when the SIP-UA registers in the IMS network, it provides itsdata towards the P-CSCF entity through which the registration is takingplace. By virtue of this P-CSCF entity sharing this subscriber data withother P-CSCF entities within the plurality of P-CSCF entities, theSIP-UA is essentially providing its data towards the plurality of P-CSCFentities. This plurality of P-CSCF entities may be considered toconstitute a logical P-CSCF entity.

Likewise, data from the SIP-UA is sent to the S-CSCF entity. The SIPmessage carrying the data from the SIP-UA to the S-CSCF entity traversesthe P-CSCF entity (that is to say, the P-CSCF entity that was selectedfor the registration procedure). The selected P-CSCF entity may addspecific data elements to this SIP message, such as Path. By virtue ofthe S-CSCF entity sharing the subscriber data with other S-CSCF entitiesin the plurality of S-CSCF entities, the SIP-UA and P-CSCF entity areessentially providing their data towards the plurality of S-CSCFentities. This plurality of S-CSCF entities may be considered toconstitute a logical S-CSCF entity.

In similar fashion, the S-CSCF entity stores for this subscriber theaddress of a logical P-CSCF entity, and the HSS stores for thissubscriber the address of a logical S-CSCF entity.

Whenever a SIP session is established or a stand-alone transaction isinitiated, the SIP request may be sent via the logical S-CSCF entity forthe subscriber and via the logical P-CSCF entity for the subscriber.

FIG. 4 shows a simplified example of SIP session establishment by theSIP-UA. It is noted that provisional responses are not depicted in thisexample.

In step (1), the SIP-UA uses the discovered P-CSCF entity address, i.e.the P-CSCF group identifier, and the received service route, i.e. theS-CSCF group identifier, to set the Route headers in the SIP Invite. DNSresolving by the SIP-UA results in the Invite being sent to one P-CSCFentity of the plurality of P-CSCF entities associated with the P-CSCFgroup identifier, in this example to P-CSCF1.

In step (2), the selected P-CSCF entity uses its own address for the Viaheader and for the Record-route header. This will have the effect thatthe remainder of the transaction (Via header) as well as the remainderof the SIP session (Record-route header) run through this P-CSCF entity.DNS resolving by P-CSCF1, using the retrieving means 33, results in theInvite being sent to S-CSCF1, in this example.

In step (3), the selected S-CSCF entity uses its own address for the Viaheader and for the Record-route header. This will have the effect thatthe remainder of the transaction as well as the remainder of the SIPsession run through this S-CSCF entity.

In step (4), the SIP-UA stores the received Record-route headers forsending subsequent requests within this SIP session, starting with theAck request.

FIG. 5 shows a simplified example of SIP session establishment towardsthe SIP-UA. It is noted that provisional responses are not depicted inthis example.

In step (1), a SIP session is established towards the S-CSCF entity ofthe SIP-UA. The address of the S-CSCF entity may e.g. be obtained fromthe HSS by the I-CSCF entity. In this example, the address will be theS-CSCF group identifier. DNS resolving of the S-CSCF group identifier(in this example by the I-CSCF entity retrieving means 38) results inthe Invite being sent to one of the S-CSCF entities of the plurality ofS-CSCF entities associated with the S-CSCF group identifier, in thisexample to S-CSCF1.

In step (2), the selected S-CSCF1 uses its own address for Via headerand for Record-route header. The remainder of the transaction and theremainder of the SIP session are routed through this S-CSCF entity.S-CSCF1 uses the stored Path header to set the Route header. DNSresolving of the P-CSCF group identifier (in this example by S-CSCF1retrieving means 48) results in the Invite being sent to one P-CSCFentity of the plurality of P-CSCF associated with the P-CSCF groupidentifier, in this example to P-CSCF1.

In step (3), the selected P-CSCF1 uses its own address for Via headerand for Record-route header. The remainder of the transaction and theremainder of the SIP session are routed through this P-CSCF.

FIG. 6 shows a simplified example of SIP session establishment by theSIP-UA, whereby data synchronization is applied for each completed SIPtransaction.

In this example, the selected P-CSCF entity and the selected S-CSCFentity use the P-CSCF group identifier and S-CSCF group identifier,respectively, to set the Record route header. As a result, subsequentrequests in the SIP session may be handled by any one of the proxyentities within the respective pluralities.

In this example, neither the data synchronization between P-CSCFentities in the plurality of P-CSCF entities nor the datasynchronization between S-CSCF entities in the plurality of S-CSCFentities will be done through SIP signalling or through Diameterprocedures. This approach results from the fact that the plurality ofP-CSCF entities is addressed as and identifies itself as a single,logical P-CSCF entity.

In this example, when a P-CSCF entity has processed a registrationprocedure from a subscriber (Register transaction successfullycompleted), it uses data distribution techniques to share subscriberdata with the other P-CSCF entities in the plurality of P-CSCF entities.This data distribution is preferably completed before the P-CSCF entityresponds to the SIP-UA with a confirmation of the registration. Asubsequent SIP session establishment or stand-alone SIP transaction maybe handled by any of the P-CSCF entities in the plurality of P-CSCFentities, as each P-CSCF entity will have (access to) the subscriberdata resulting from the registration.

The P-CSCF entities in the plurality may each have a database.Alternatively, the P-CSCF entities may have access to a shared database.The same principle can be applied to the S-CSCF entities. A shareddatabase is a common technique for redundant computer systems.

Likewise, S-CSCF entities in the plurality of S-CSCF entities use datadistribution techniques to share subscriber data with the other S-CSCFentities in the plurality.

Planned server shutdown is described for a P-CSCF entity. It will beappreciated that the principle applies equally for a S-CSCF entity.

When a P-CSCF entity is to be put into maintenance, DNS 16, 16′ could bemodified such that DNS will from that moment on no longer return thehost name and/or IP address for that particular P-CSCF entity. Due tocaching by DNS clients, a defined grace period is preferably applied.After that grace period and resulting from the DNS modification, no nodewill select this particular P-CSCF entity anymore, when sending a SIPrequest message to the P-CSCF group identifier. Hence, the load willgradually be distributed over n−1 P-CSCF entities, i.e. the load on theother P-CSCF entities in the group will gradually increase.

When a particular P-CSCF entity is selected at SIP sessionestablishment, this P-CSCF entity may be used for the entire SIPsession. It is needed to monitor the SIP sessions in progress on theP-CSCF entity that is to be put in maintenance mode. All sessions beinghandled by this P-CSCF entity are preferably completed before the P-CSCFentity can be put into maintenance mode. By that time, all load relatedto the subscriber group is taken over by the other P-CSCF entities inthe plurality of P-CSCF entities. The P-CSCF entity may now be put inmaintenance mode.

When the concept of a logical P-CSCF entity is applied to a single SIPtransaction instead of to a single SIP session, the above-referredadditional grace period (time needed to monitor the P-CSCF entity foractive transactions) will be much shorter, since the duration of atransaction is smaller than the duration of a SIP session.

When the P-CSCF entity is to be put back into operation, the followingsteps could be taken in this order (apart from other required stepsneeded to get the P-CSCF entity fully operational): the P-CSCF entityshall instruct its database to initiate data synchronization with thedatabase(s) from the other P-CSCF entities in the plurality of P-CSCFentities; and when data synchronization is complete, DNS may be modifiedback, so nodes may select this particular P-CSCF entity when sending aSIP request message to the P-CSCF group identifier. Due to caching IPaddressing in other nodes (specifically the SIP-UA and the S-CSCFentity), the load on this P-CSCF entity will gradually increase and theload on the other P-CSCF entities will gradually decrease.

In general, when, in the case of node failure or node shut down, theload from n P-CSCF entities or m S-CSCF entities needs to be taken overby n−1 P-CSCF entities or m−1 S-CSCF entities, respectively, these n−1P-CSCF entities or m−1 S-CSCF entities preferably have sufficientcapacity hereto. The network (node capacity and link capacity) ispreferably dimensioned accordingly.

Below, node failure is described for a P-CSCF entity. It will beappreciated that the principle applies equally for a S-CSCF entity.

It is considered that in the case of node failure, ongoing transactionsbeing handled by a P-CSCF entity may be not completed and ongoing SIPsessions may fail, presuming that data related to a SIP session is notsynchronized during the SIP session. When a subsequent (i.e. within aSession) transaction request is directed to a particular P-CSCF entitywhilst that particular P-CSCF entity has become inoperational since theestablishment of the session, that transaction can't be handled byanother P-CSCF entity. The unforeseen failure of transaction relatedsignalling will have the effect that the transaction as such fails. Inaddition, if a SIP session is to be handled in its entirety by the sameP-CSCF entity as where the SIP session establishment started, thenP-CSCF entity failure has the effect that any SIP session being handledby that P-CSCF entity will fail (e.g. due to absence of heartbeatresponse messages).

When an entity establishes a SIP session or (stand-alone) SIPtransaction towards the plurality of P-CSCF entities and this entityuses the (cached) host name/IP address of a P-CSCF entity that has nodefailure, then the SIP session establishment attempt or SIP transactionattempt will fail. DNS methodology may be applied in order to have thisentity attempt to establish this SIP session/SIP transaction through anyof the other P-CSCF entities. For example, when DNS query is applied forthe plurality of P-CSCF entities, the DNS client may receive a set of IPaddresses.

When the P-CSCF entity failure is detected, DNS may be modified asdescribed above for planned server shutdown. Entities will thengradually no longer attempt to contact this P-CSCF entity, therebyreducing the negative effect of the node failure. New SIP sessions maythen be handled by the remaining n−1 P-CSCF entities.

When the node is operational again, the steps for putting node back inoperation, as described above for planned server shutdown, may beapplied.

In the foregoing specification, the invention has been described withreference to specific examples of embodiments of the invention. It will,however, be evident that various modifications and changes may be madetherein without departing from the broader spirit and scope of theinvention as set forth in the appended claims.

In the preceding has been described that a group of P-CSCF entities mayact as a logical P-CSCF entity. It has also been described that a groupof S-CSCF entities may act as a logical S-CSCF entity. It will beappreciated that it is also possible that a group of P-CSCF entitiesacts as a logical P-CSCF entity while use is made of a single S-CSCFentity. It will be appreciated that it is also possible that a group ofS-CSCF entities acts as a logical S-CSCF entity while use is made of asingle P-CSCF entity.

It will be appreciated that the group of P-CSCF entities acting as thelogical P-CSCF entity and/or the group of S-CSCF entities acting as thelogical S-CSCF entity may be used for the establishment and continuationof a SIP session and/or the initiation of a stand-alone SIP transaction.

It will be appreciated that when a SIP session is established, any newSIP transaction initiated within this SIP session uses a signalling paththat is the result of individual SIP proxies adding a Record-routeheader to the SIP Invite (not considering Back-to-back user agent). Whena SIP proxy, e.g. a P-CSCF entity, adds a Record-route header to the SIPInvite, it may use the P-CSCF group identifier. This has the effect thatsubsequent transactions within this SIP session may be establishedthrough any of the P-CSCF entities within the group of P-CSCF entitiesassociated with the group identifier. Alternatively, the P-CSCF entitymay use its own name (address) as Record-route header. In that case,subsequent transactions within this SIP session will be establishedthrough this specific P-CSCF.

It will be appreciated that when subsequent transactions within a SIPsession are established through any one of the P-CSCF entities withinthe group, the P-CSCF entity preferably ensures that datasynchronization takes place, and preferably is completed, no later thanthe moment that the P-CSCF entity forwards the last Request or Responsemessage related to this transaction. It will be appreciated that oncethe P-CSCF entity has forwarded the last Request or Response messagerelated to this transaction, a subsequent transaction may be initiatedthat may be handled by another P-CSCF entity within the group. Thatother P-CSCF entity needs to have (access to) the up-to-date subscriberdata.

It will be appreciated that the described method of data synchronizationper transaction, if desired, may also be applied to record routingproxies that do not have subscriber data, such as Interconnection BorderControl function (IBCF) entities. SIP transactions within one SIPsession may then be spread over available IBCF entities within a groupof IBCF entities.

However, other modifications, variations, and alternatives are alsopossible. The specifications, drawings and examples are, accordingly, tobe regarded in an illustrative rather than in a restrictive sense.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word ‘comprising’ does notexclude the presence of other features or steps than those listed in aclaim. Furthermore, the words ‘a’ and ‘an’ shall not be construed aslimited to ‘only one’, but instead are used to mean ‘at least one’, anddo not exclude a plurality. The mere fact that certain measures arerecited in mutually different claims does not indicate that acombination of these measures cannot be used to advantage.

What is claimed is:
 1. A method performed by a User Agent (UA) ofregistering in an Internet Protocol Multimedia Subsystem (IMS)communications network, the UA using a Proxy Call Session ControlFunction (P-CSCF) entity with a Serving Call Session Control Function(S-CSCF) entity, the method comprising: determining, by the UA, a firstgroup identifier associated with a plurality of P-CSCF entities;selecting, by the UA and based on the first group identifier, a networkaddress corresponding to one of the plurality of P-CSCF entities;sending, by the UA, to the selected P-CSCF entity using the networkaddress, a message to register with each of the plurality of P-CSCFentities associated with the first group identifier; and establishing,by the UA, a session initiation protocol (SIP) session through adifferent one of the plurality of P-CSCF entities associated with thefirst group identifier.
 2. The method of claim 1, further comprising:using the selected P-CSCF entity throughout a SIP session.
 3. The methodof claim 1, wherein selecting includes selecting the P-CSCF entity pertransaction within a SIP session.
 4. The method of claim 3, furthercomprising: sending, by the UA, to the selected P-CSCF entity, a messageincluding the first group identifier.
 5. The method of claim 4, furthercomprising: selecting, by the UA and based on a second group identifier,a network address corresponding to one of the plurality of S-CSCFentities; and receiving, by the UA, from the selected S-CSCF entity aresponse to the message.
 6. The method of claim 3, wherein selecting theP-CSCF entity using the first group identifier includes performing aDomain Name Server (DNS) query using the first group identifier.
 7. Themethod of claim 6, further comprising: receiving, by the UA, a list ofnetwork addresses associated with the first group identifier in responseto the DNS query.
 8. A method performed by a User Agent (UA) ofregistering in an Internet Protocol Multimedia Subsystem (IMS)communications network, the UA using a Proxy Call Session ControlFunction (P-CSCF) entity with a Serving Call Session Control Function(S-CSCF) entity, the method comprising: discovering, by the UA, a secondgroup identifier associated with a plurality of S-CSCF entities;selecting, by the UA and based on the second group identifier, a networkaddress corresponding to one of the plurality of S-CSCF entities;sending, by the UA, to the selected S-CSCF entity using the networkaddress, a message to register with each of the plurality of S-CSCFentities associated with the first group identifier; and establishing,by the UA, a session initiation protocol (SIP) session through adifferent one of the plurality of S-CSCF entities associated with thefirst group identifier.
 9. The method of claim 8 further comprisingthereafter using the selected S-CSCF entity throughout a SIP session.10. The method of claim 8 further comprising, thereafter repeating theselecting the S-CSCF entity per transaction within a SIP session. 11.The method of claim 10 further comprising thereafter including thesecond group identifier in the message transmitted by the UA towards theselected S-CSCF entity.
 12. The method of claim 11 further comprising:selecting, by the UA and based on a first group identifier, a networkaddress corresponding to one of a plurality of P-CSCF entities; andreceiving, by the UA, from the selected S-CSCF entity via the selectedP-CSCF entity, a response to the message.
 13. The method of claim 10,wherein selecting the S-CSCF entity on the basis of the second groupidentifier includes performing a Domain Name Server (DNS) query usingthe second group identifier.
 14. The method of claim 11, furthercomprising: receiving, by the UA, a list of network addresses associatedwith the second group identifier in response to the DNS query.